[YottaDB/DB/YDB#619] Fix CallMT and CallMDescT usages in util.go and util_test.go now that they return negative error codes

* `CallMT` and `CallMDescT` are just Go wrappers for `ydb_ci_t()` and `ydb_cip_t()`. The latter functions
  were changed as part of YottaDB/DB/YDB#619 to return negative error codes (instead of positive error
  codes) and therefore the `CallMT` and `CallMDescT` functions too inherit that change.

  A few usages of these functions in `util.go` and `util_test.go` were fixed to incorporate the new behavior.